If the parsnips and carrots are very thick, cut them in half lengthwise. Slice each one diagonally in 1-inch-thick slices. The vegetables will shrink while cooking, so don’t make the pieces too small. Place the cut vegetables on a sheet pan. Add the olive oil, salt, and pepper and toss well. Roast for 20 to 40 minutes, depending on the size of the vegetables, tossing occasionally, until the parsnips and carrots are just tender. Sprinkle with dill and serve hot.